A photoelectric conversion element (organic photodiode) using an organic material can photoelectrically convert only a specific color (wavelength band). And because of such a characteristic, when the photoelectric conversion element is used as a photoelectric conversion element in a solid-state imaging device, it is possible to obtain a structure having sub-pixels laminated, not possible with a conventional solid-state imaging device in which each of sub-pixels is formed of a combination of an on-chip color filter (OCCF) and a photoelectric conversion element and the sub-pixels are arranged two-dimensionally. Therefore, the photoelectric conversion element can receive incident light with a high efficiency, and therefore a higher sensitivity of the solid-state imaging device can be expected. In addition, there is an advantage that a false color is not generated because demosaic processing is not required.
An organic photodiode used in an imaging device or an imaging element has a structure the same as or similar to various organic thin-film solar cells. Conventionally, as a structure of the organic photodiode, a structure using a p-n junction or a p-i-n junction (for example, refer to JP-2006-33942 A), a structure using a bulk heterostructure (refer to JP 2007-123707 A), and a structure using a buffer layer (for example, refer to JP 2007-311647 A and JP 2007-088033 A) are known, and are exclusively intended to improve a photoelectric conversion efficiency.